This invention relates to a contact for a vacuum circuit breaker which has a high breakdown voltage and excellent large current characteristics.
The contact for the vacuum circuit breaker has to satisfy the following characteristic requirements:
(1) The interrupting performance should be high.
(2) The breakdown voltage should be high.
(3) The contact resistance should be low.
(4) The fusing force should be low.
(5) Wear should be low.
(6) The chopping current should be low.
However, it is difficult to meet all these requirements with an actual contact, and generally contacts which can meet only particularly important characteristics are used for specific applications while more or less sacrificing the other characteristics.
Heretofore, copper-chromium alloys (hereinafter referred to as Cu-Cr, similar element symbol expression being used for other elements and alloys of elements as well), Cu-Co, Cu-Bi, Cu-Cr-Bi, Cu-Co-Bi, etc., have been used for vacuum circuit breaker contacts. As a result of experiments conducted by the inventors, however, it is found that contacts which do not contain a low-melting metal such as Cu-Cr, and the like have a disadvantage that the fusing force is somewhat high even if they have a good interrupting performance, while contacts containing a low-melting metal, such as Cu-Bi and the like also have such disadvantages that the chopping current is somewhat high if the content of the low-melting metal is less than or equal to 1% by weight, and the interrupting performance and breakdown voltage are sacrificed if the content of the low-melting metal is more than 1% by weight.
Conventional contact alloys have been composed of Cu which is a good electric conductor, and such elements as Cr, Co, Bi and the like, which do not form a solid solution with Cu in order to prevent the reduction of electric conductivity. As a result, when these alloys are produced by a melting technique, it results in precipitation type metal structures having large size coarsely distributed grains. Generally, the finer and the more uniform the contact alloy in the metal structure, the better the interrupting performance, the breakdown voltage, and the chopping current. For this reason, the alloy obtained by the melting process is usually subjected to a heat treatment or to pulverization followed by sintering treatment in order to obtain an alloy which is uniform and fine in metallic grain structure. On the other hand, in the case of the powder sintering technique, an alloy having a uniform, fine grain metallic structure is obtained by previously employing powders having small particle sizes as the raw material.
However, these prior art contact alloys have limitations on their breakdown voltage, large current characteristics, chopping current, and uniformity and fineness of their metal structure. Thus, strong need exists for a contact alloy having better characteristics.